In the prior art, various types of optical control devices such as an optical modulator, an optical switch, a polarization device, and the like have been practically used. Examples of an optical waveguide formed in the optical control device include a ridge waveguide in which convex-shaped ridges are formed on a surface of a substrate, and a waveguide in which impurities such as Ti are diffused by heat.
Recently, in order to lower a drive voltage or improve high frequency response characteristics of the optical control device, a substrate constituting the optical control device has been formed into a thin plate, or an optical circuit in the optical control device has decreased in size.
The decrease in size of the optical circuit results in a decrease in cross-sectional area of the optical waveguide constituting the optical circuit. Accordingly, the following problems arise.
(1) An acceptable value in manufacturing the optical waveguide structure becomes small, thereby configuring the waveguide as the multi-mode waveguide.
(2) A coupling loss between the known single-mode optical fiber and the optical control device (optical circuit) increases due to the (1), thereby degrading a reproducibility of an optical signal.
FIG. 1 is a schematic sectional view of the optical control device in which a convex-shaped ridge is formed on a substrate and concave sections (hereinafter, referred to as “trench”) are formed on both sides of the ridge to form a ridge waveguide. A reinforcing plate 3 is adhered to a substrate 1 with an adhesion layer 2 interposed therebetween. A reference numeral 4 denotes the ridge optical waveguide.
The propagation mode of the optical waveguide is changed in accordance with the shape of the ridge waveguide shown in FIG. 1. Specifically, as shown in FIG. 2, the propagation mode of the optical waveguide is divided into single-mode and multi-mode areas depending on the condition of a width W and a depth D of an optical waveguide 4 and a thickness H of the substrate.
Additionally, FIG. 2 shows a graph modeling the ridge waveguide without the trench. Suppose that a tilt angle θ of the side surface of the waveguide 4 is 70°, a propagating light wave is 1.55 μm in a wavelength λ and is in a TM mode, a refractive index of the substrate is 2.1, the refractive index of an upper surface of the substrate is 1.0 like air, and the refractive index of a lower surface is 1.45 like SiO2.
As shown in FIG. 2, as the height H of the substrate becomes thin, the value of a horizontal axis (r=(H=D)/H) decrease and the value of a vertical axis (W/H) increase. Accordingly, an overall change in the propagation mode from the single-mode to the multi-mode can be easily understood.
Meanwhile, as disclosed in Patent Document 1, an acceptable value of the single-mode waveguide can be increased by forming a slab waveguides beside the optical waveguide. However, the multi-mode of the propagation mode occurring at the time when the optical circuit decreases in size as shown in FIG. 2 also occurs in the optical control device having the ridge waveguide with the trench. When the substrate is formed into a thin plate, particularly 10 μm or less, it is necessary to configure the trench to have a width (symbol “T” in FIG. 1) equal to or less than of 1 μm in order to realize the single mode propagation.
[Patent Document 1] JP-A-2004-219751
Additionally, as for the ridge waveguide using the thin plate, a difference Δn in the refractive index between the substrate and air or between the substrate and the adhesion layer (buffer layer) is large. Accordingly, in order to maintain in the single-mode, the cross-sectional area of the optical waveguide itself is required to become small. When the cross-sectional area of the waveguide becomes small, a manufacturing error involved in the width of the optical waveguide and the depth of the trench which can be ignored in the prior art cannot be ignored any more. When the optical waveguide is to be operated in the multi-mode, an S/N ratio or an optical insertion loss is degraded. Further, when the ridge waveguide has a Y branch area and the reproduction of the shape is unsatisfactory, the S/N ratio or the optical insertion loss is degraded as well. Even when the conditions of the single mode in the light input and output sections of the optical waveguide are satisfied, a little different shape results in a considerable change in the optical insertion loss.
Meanwhile, when the ridge waveguide is manufactured, a mask corresponding to a pattern of the optical waveguide is placed on a substrate having an electro-optic effect, and then a portion of the substrate is removed by performing the wet etching process, the dry etching process, or the like, or a groove or the like is formed by performing a machining process such as the dicing saw process and the like. However, when the trench of the ridge waveguide is formed by the known wet or dry etching process, a lower portion of a mask 10 disposed on a substrate 1 becomes an undercut 11 shown in FIG. 3A or becomes a locally unusual shape 12 shown in FIG. 3B. The optical waveguide with such shapes is configured as a leaky mode (a mode in which light does not propagate and since there are uneven portions in the waveguide shape, the mode refers to a state light does not diffuse) or the multi-mode waveguide. Accordingly, it is difficult to manufacture the single-mode waveguide. Further, in the machining process, the limitation of the groove width is 2 μm or so, and particularly when the machining process is performed in order to manufacture the thin plate with the thickness of 10 μm or less, the substrate may be damaged.
Accordingly, in order to prevent the S/N ratio or the optical insertion loss from being degraded in the optical control device using the thin plate, a high process precision of 0.1 μm or less is required when the trench of the ridge waveguide is processed. However, it is difficult to obtain the satisfactory precision by means of the known etching or machining process.